97 Monte Carlo No A/C

Hello,
I have a 1997 Chevy Monte Carlo LS, 3.1L, Automatic, with 150,000 miles on it. The car has a A/C leak for about a year, that nobody can find it. I have been using over the counter A/C refills. The problem is when I tried to use the over the counter refill this time the car has not been producing cold air. There is a guage on the refill and it shows that the system is at half way in the green area. Maybe I have been using the wrong port. What port does the refil go to, both of my caps are black. Thank You for your help.